If I write POS in the margin of your essay, you have either (a) added an apostrophe to a word that is not possessive, or (b) neglected to include the possessive apostrophe in a word that is possessive. Add or remove apostrophe as needed.
Make sure that you put the apostrophe in the right place for plural possessives.
Note that “it’s” means “it is.” The possessive of “it” is “its.” It’s/its confusion errors are my pet peeve and will probably earn you a marginal unhappy face. 😦